Machine Vision Improves Surgical Tubing Quality

Surgical tubing is extruded at rates on the order of 600
feet per minute. It’s critical to detect inclusions and other flaws that can
cause blockages in the tubing. A laser inspection system was used in the past
but the tubing manufacturer was concerned that it was missing many inclusions.
The manufacturer evaluated the Integro Anaconda tube inspection system which is
based on Cognex machine vision technology
and provides very sensitive flaw detection at production speeds. When the
machine vision system was first installed on a trial basis, it generated alarms
every few minutes.

The Integro Anaconda tube inspection system installed.

At first the manufacturer thought there was a problem with
the inspection system, but manual inspection of the tubing showed that each
alarm represented an inclusion that had been overlooked by the laser inspection
system. “The vision system showed that the old inspection system had been
missing many inclusions,” said Shawn Campion, Vice President, Integro Technologies.
“The tubing manufacturer decided to convert their manufacturing process to
vision inspection. The system allowed them to implement an active cutting
process that removes the bad material and starts a new run from that point.
Quality has been dramatically improved. To the best of our knowledge, the
machine vision system has not missed a single flaw.”

Surgical tubing inspection challenges
The production of surgical tubing begins when plastic pellets or chips are
heated and then pushed under pressure by a screw through a die that forces the
molten plastic into the shape of the finished tube. The extruded tube is cooled
in a vat of water and wound onto drums. Surgical tubing requires the most
stringent quality control during the production process but the high rate at
which surgical tubes are extruded places enormous demands on any in-line
inspection system.

In the past, the tubing manufacturing used a laser scanning
inspection process in which the object to be measured is scanned with two
parallel laser beams that cross the measuring field at a right angle to each
others. The shadow cast by the laser beams produces a light/dark signal that is
acquired by a measuring head and analyzed to determine the shadow cast by the
extruded material in order to measure inside and outside diameter. While this
method is capable of accurate measure of product dimensions it has only limited
ability to detect flaws such as inclusions.

The top plate of the Anaconda system.

Laser-based inspection is currently the standard method used
in the surgical tubing industry but the tubing manufacturer was concerned about
its ability to detect product flaws. For this reason, the tubing manufacturer
invited Integro Technologies to demonstrate its tubing inspection system which
uses machine vision to provide product measurement as well as flaw detection.
While machine vision systems are inherently more sensitive than the much
simpler laser method, the challenge in applying machine vision to this
application has long been the need to keep up with the exceptionally high
production speeds used to produce surgical tubing.

Achieving high speed vision inspection
Integro engineers addressed the challenge of keeping up with the high speeds at
which surgical tubing is produced through a unique combination of hardware and
software. Integro engineers paired a high speed camera with the Cognex
MVS-8602e digital frame grabber which supports the Camera Link communications
protocol that allows machine vision frame grabbers and cameras to exchange data
at very high speeds. The MVS-8602e supports two independent image acquisition
channels that allow the connection of two area scan, line scan or a combination
of both types of Camera Link cameras in Base configuration. Alternatively, one
area scan or line scan camera can be used in Medium configuration. The frame
grabber uses the PCI Express (PCIe) x4 bus which utilizes four 250 MB/s PCIe
bus links to quickly transfer image data from the frame grabber to the host
memory. The frame grabber also features Direct Memory Access (DMA) channels,
pipelined process and on-board image buffers to allow images to be transferred
while the system is acquiring new images.

Integro engineers generated further speed improvements by
partitioning the vision processing code into different sections of code called
threads that run in parallel on the separate cores of a four-core workstation.
The engineers developed the multi-threaded vision processing routines using
Cognex VisionPro software which automatically creates separate threads for
image acquisition and vision processing. VisionPro is a suite of machine vision
software tools that supports the Microsoft Visual Studio® .NET programming
environment.

The Anaconda HMI; a screen capture from an Anaconda
inspection.

In addition to application-level optimization, the machine
vision tools are also optimized by parallelizing their algorithms so they use
multiple cores simultaneously. Parallelization is most helpful for image
processing filters and other vision tools that run local operations on small
regions of the image such as median Gaussian and morphology operations. The
image is divided into different pieces and each one is assigned to a different
thread. The results from each thread are then combined to produce the final
results. The final speedup depends on the algorithm and the number of cores.
Overhead creates small inefficiencies so even well optimized vision tools do
not run four times faster on a four core workstation.

The vision platform also adapts to the number of cores in
the system which is important since the number of cores available may change
over time. This allows applications such as Integro’s, which was written for a
four-core workstation, to run efficiently on an eight-core workstation without
touching the source code or recompiling. Downstream maintenance savings are
provided while offering the capability to upgrade performance simply by
deploying the system on a workstation with more cores.

An image from an Anaconda inspection showing tube occulsions
and gels.

Integro engineers used the Cognex VisionPro tool library to
provide 100% inspection of the product’s inside and outside dimensions and flaw
detection at production speeds up to 1000 feet per minute. The system provides
process data comparable to laser systems and also provides storage of failed
images for utilization in troubleshooting and process refinements. The system
is configured to track defects and provide time-based delays to fire outputs
for active cutting. Integro engineers took advantage of the flexibility of the
machine vision platform to write an application that automatically determines
the key characteristics of a tube that is placed in front of the camera and
sets itself up with inspection thresholds. Users with sufficient privileges can
modify parameters for the given product to optimize system performance and
sensitivity.

Substantial quality improvements
In the past, the surgical tubing manufacturer produced a drum with 1000 feet of
material that was cut to the size of the customer’s order after the fact. The
problem is that there was no way to guarantee the quality of each order. Now,
when the much more sensitive vision system detects a flaw, the vision system
fires an output and a section three feet long centered on the flaw is cut out
and discarded. If the flaw is in the middle of a customer order of 25 feet, for
example, then a new 25 foot section is run after the section is cut out. This
approach makes it possible to provide each customer with a certificate that
their order has been inspected with a vision system and found to be free of
defects. The tubing manufacturer’s customers have been so happy with the
improvements in quality that the manufacturer has been able to deliver using
this approach that several of them have rewritten their purchasing
specification to require that vision inspection be used on all of their
surgical tubing orders.

“The system has dramatically improved the quality of the
surgical tubing manufacturer’s product,” Campion concluded. “To the best of my
knowledge, the vision system has never missed a defect. The higher sensitivity
of the machine vision system has also provided feedback that has helped the
tubing manufacturer substantially reduce the number of defects. The cost of the
vision system is higher than laser inspection but feedback from the tubing
manufacturer indicates that the system quickly paid for itself through quality
improvements.”

Jerry Fireman is the President of Structured Information and has spent 30
years writing about nearly every type of technology. To date, he has
written more than 9,000 articles that have been published in over
3,000 trade journals, technical journals and mass media around the world.